A comparative study of electro-dewatering process performance for activated and digested wastewater sludge

Abstract

Electro-dewatering (EDW) is an alternative emerging and energy-efficient technology that provides improved liquid/solids separations in the dewatering of wastewater sludge. The EDW technology is not only an innovative dewatering method for significantly reducing the volume of wastewater sludge before re-utilization or disposal, but is also a promising emerging method which may potentially be used for decontamination purposes. In this study, the influence of the sludge properties (e.g. electrical conductivity, zeta potential, specific cake resistance, among others) on their mechanical and electrical behaviour in terms of dewaterability and electro-dewaterability, the applied current (current density from 20 to 80 A/m2), and filter cloth position relative to the electrode was investigated. A two-sided filter press at lab-scale with moving anode was used, and the treatment performance of the EDW process on two different types of wastewater sludge (activated and digested) was thoroughly assessed from both an electrochemical viewpoint and in terms of the dewatering rate. The results showed that the conditioned digested sludge was more easily dewatered by mechanical dewatering (MDW) with 34–35% (w%) of dry solids content compared to 19–20% (w%) for the activated sludge, thanks to the lower content of both the microbial extracellular polymeric substances (EPS) and the volatile suspended solids fraction.For the EDW results, the electrical conductivity of the sludge was pivotal to the dryness of the final solids and therefore also to the dewatering kinetics. The results demonstrated that the activated sludge arrived at an equilibrium much faster (after approximately 3600 s) compared with digested sludge, thanks to its lower electrical conductivity (0.8 mS/cm) providing a greater voltage drop across the cathode and therefore more repulsion of the solids from the cathode leading to continuously high filtrate flowrate. Also the EDW performance was analysed by comparing the ratio of the filtrate volume collected at the anode to the volume collected at the cathode side. For digested sludge at 5 bar, 40 A/m2 different positions of the filter cloth were tested but these configurations barely impacted the EDW performance, despite having a significant impact on the energy requirements. At industrial scale, it would be useful to position the filter cloths at some distance from the electrodes, but this study shows that this benefit may be quickly outweighed by the loss in EDW energy efficiency.